The present invention relates to ultrasonic level or flow detectors that have the capability of automatically adjusting the frequency at which ultrasonic energy is transmitted. The present invention also relates to ultrasonic detectors that have the capability of discriminating between echo signals based upon their frequency.
Ultrasonic detectors may be used to determine the level of a liquid in a tank or the magnitude of the flow of liquid within a conduit. Such detectors have transducers that emit bursts of ultrasonic energy and respond to echoes of the bursts that are reflected by various surfaces. For example, a detector that determines the level of a liquid in a tank transmits energy bursts towards the liquid surface and detects when echoes of the energy bursts are received. The detector then determines the liquid level based upon the time difference between the transmission of an energy burst and the receipt of the corresponding echo. This travel time of an energy burst is directly related to the distance from the transducer to the liquid surface.
Ultrasonic detectors typically have piezo-ceramic transducers that convert trains of electrical pulses into ultrasonic energy bursts. The transducers also convert the echoes they receive back into electrical signals. The efficiency of an ultrasonic transducer at converting the electrical pulse trains to energy bursts varies with the frequency at which the transducer is excited. It is known that such transducers optimally convert electrical signals to ultrasonic energy at their natural frequency of oscillation, which is referred to as their "ringdown" frequency. Any substantial departure from exciting a transducer with an electrical pulse train having a frequency that matches the ringdown frequency of the transducer may adversely affect the operation of the detector.
A conventional ultrasonic detector typically includes a pulse generator that generates the electrical pulse trains which excite the transducer. The frequency at which the pulse generator oscillates may be manually adjusted, for example with a potentiometer. Once the frequency is manually adjusted, the pulse generator generates pulse trains whose frequency is fixed.
After manufacture of an ultrasonic detector, the frequency of the pulse generator is manually adjusted by an operator so that it substantially matches the ringdown frequency of the transducer. This frequency adjustment is necessary since the ringdown frequency of one transducer may vary by about 5-10% from the ringdown frequency of another transducer.
This manual adjustment of the transducer excitation frequency has a number of disadvantages. One disadvantage is the time required to perform the frequency adjustment. Another disadvantage is that the ringdown frequency may drift with temperature and the age of the transducer. Thus, although the transducer excitation frequency is initially set correctly, it will not be set correctly when the temperature changes significantly and when the transducer ages. A third disadvantage is that the detector must be retuned when the transducer is replaced with a new transducer.
Another problem with ultrasonic detectors in general is the lack of capability of distinguishing true echo signals from spurious echo signals. A "true" echo signal is one generated by the transducer in response to receiving an echo from a surface, the position of which is to be determined. For example, if the detector is determining the liquid level within a tank, all echo signals generated by the transducer in response to the reflection of energy bursts from the liquid surface would be "true" echo signals. A "spurious" echo signal is not generated by the reflection of ultrasonic energy. Instead, it is an electrical signal induced in the detector as a result of electrical interference, or noise, that might be generated, for example, by the startup of a pump adjacent the tank. The erroneous detection of spurious echo signals may adversely affect the ability of the detector to accurately locate the liquid surface.